I have been looking into the differences between recreational oxygen and medical oxygen. It seems like you really do not know what you are getting when purchasing a recreational oxygen machine or "oxygen bar" as they are sometimes called. They rarely list the output capacity or supply any details about what you are paying for. Too small an oxygen doze is not going to have any substantial effect.
The medical oxygen concentrators do provide you with details since the government regulates this category. Medical oxygen concentrators require a prescription in order to purchase. The top brands include Respironics, Invacare, Inogen, Drive Medical, John Bunn and Sequal. A chart comparing oxygen concentrators can be seen at the bottom of the following web page: Vitality Medical Oxygen Concentrator Comparison. There are actually two charts on this webpage. The first chart compares 5 liter oxygen machines and the second compares 10 liter machines.
A chart comparing economy oxygen concentrators can be seen at Oxygen Concentrator Review.
Medical oxygen concentrators appear to be a better option as a treatment device.
Sorry, I could not find a comparison chart for recreational oxygen machines.